1. Field of the Invention
This invention relates to a process for manufacturing a radiographic intensifying screen (hereinafter referred to as "intensifying screen").
2. Description of the Prior Art
Radiography is generally classified into two types, viz., medical radiography used for medical diagnosis and industrial radiography used for nondestructive inspection of industrial materials. In both types, the intensifying screen is used in face contact with a radiographic film to increase the sensitivity of the radiographing system. The intensifying screen is essentially composed of a support and a fluorescent layer formed thereon. The fluorescent layer is composed of a phosphor which emits light of high luminance by the excitation of radiation (hereinafter referred to as "radioluminescent phosphor") dispersed in a resinous binder. The fluorescent layer is usually covered with a transparent protective layer. Some intensifying screens have a reflective layer or an absorptive layer between the support and the fluorescent layer. Further, some intensifying screens used for nondestructive inspection of industrial materials have a metallic foil between the support and the fluorescent layer.
The intensifying screen having the above-mentioned structure has been manufactured heretofore by the following manufacturing process. That is, a radioluminescent phosphor is mixed with a proper amount of non-curable or thermoplastic resinous binder such as derivatives of cellulose (cellulose acetate butyrate, cellulose triacetate, cellulose nitrate, etc.), vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, vinylidene chloride-acrylonitrile copolymer, polyvinyl butyral, polyamide resin, butadiene-acrylonitril copolymer, styrene-butadiene copolymer, polyester resin, urethane resin or the like. Then, a proper amount of solvent is added to the mixture to prepare a dispersion having the optimum viscosity. Additives such as a dispersing agent for improving the dispersibility of the radioluminescent phosphor, and a plasticizer such as dibutyl phthalate, methylphthalyl ethyleneglycol or the like for increasing the plasticity of the intensifying screen obtained may be added to the coating dispersion. The dispersion thus prepared is applied to a support by means of a roll coater, a knife coater or the like to form a coating, and the coating is dried to form a fluorescent layer. When the intensifying screen having a reflective layer, an absorptive layer or a metallic foil between a support and a fluorescent layer is manufactured, the reflective layer, the absorptive layer or the metallic foil is provided on the support beforehand, and then the fluorescent layer is formed thereon in the same manner as described above. In this specification, unless otherwise indicated, the expression "support" means a support on which a reflective layer, a absorptive layer or a metallic foil is provided beforehand, as well as a sole support. After the formation of the fluorescent layer, a transparent protective layer is usually provided on the fluorescent layer.
As described above, in the conventional intensifying screen, a thermoplastic resin is used as a resinous binder. Recently, the intensifying screen has been required to have high degree of physical durability and chemical resistance with increase of use thereof owing to, for example, the advance in high speed serial radiography. Therefore, it has been attempted to use a curable resin which is superior in both the physical durability and the chemical resistance to the thermoplastic resin as a resinous binder of the intensifying screen instead of the thermoplastic resin. As the curable resins the practical use of which has been tested heretofore are known as two-pack resin composition and a heat-curable resin. However, when the two-pack resin composition or the heat-curable resin is used as a resinous binder, it is difficult to manufacture the intensifying screen exhibiting high radiographic image quality (hereinafter referred to as "image quality"). That is, when the two-pack resin composition is used, a dispersion having high dispersibility of the radioluminescent phosphor cannot be obtained, because a curing component or a curing catalyst of the two-pack resin composition should be mixed at the last step of the preparation of the dispersion in a short period of time to prevent the dispersion to cure during the preparation thereof. Thus, the intensifying screen exhibiting high image quality cannot be obtained. Further, when the two-pack resin composition is used, the dispersion prepared should be applied to a support as soon as possible, because the curing component or the curing catalyst has already been added thereto. In the dispersion which is left to stand for a long period of time after the preparation thereof, a curing reaction advances and accordingly, the dispersion cannot be applied to the support with high reproducibility. Further, in the dispersion which is left to stand for a still longer period of time, a gelation occurs and accordingly, the dispersion cannot be applied to the support at all. As described above, when the two-pack resin composition is used as a resinous binder, there are defects that a large quantity of dispersion cannot be prepared at a time and that a coating prepared cannot be applied to a support with high reproducibility, as well as the defect that the intensifying screen exhibiting high image quality cannot be obtained.
On the other hand, when the heat-curable resin is used as a resinous binder, the radioluminescent phosphor particles by a convection occurs in the dispersion applied to a support during the curing thereof by heating and accordingly, the dispersibility of the radioluminescent phosphor in the fluorescent layer formed is deteriorated. Thus, the intensifying screen exhibiting high image quality cannot be obtained. Further, when the heat-curable resin is used as a resinous binder, a large-scale heating apparatus for curing is required. For the above-mentioned reasons, it has been difficult heretofore to use a curable resin as a resinous binder of the intensifying screen.